Stereo Music Waveform

You can buy a stereo amplifier delivering thirty watts per channel. Or one that delivers 500 watts per channel. So, how much power do you need?

Forgive me if I start with an anecdote from my teen years. My father, a military man, had been posted to Singapore for a couple of months. I was into hifi and I handed over my hundred-dollars-plus in savings – this was a very long time ago – asking him to purchase, if possible, an amplifier for me. I was hoping for something like a Kenwood 10 watts per channel model. When he returned, he brought with him an amplifier of a brand of which I’d never heard. It was rated at an astonishing 40 watts per channel, of which I was a little sceptical, even in my teen naivety.

But it turned out that Rotel was a legitimate brand – and soon to become widely known and praised – and when Australian HI-FI tested a sample a few months later, the Rotel RA-810 proved good for north of fifty watts. Suddenly I was the envy of my equally nerdy hifi friends.

One day I took the Rotel and my compact three-way speakers into school for some event, put the speakers on the stage, wired everything up and … it might as well have been a battery-powered transistor radio on the stage. The weak, thready sound was entirely unimpressive.

But then, at a later stage, some band was in to perform for a “Social”. I plugged the Rotel into their speakers, and it filled the assembly hall with music, loudly and effortlessly.

Yes, more is better, but it should not be the only consideration

Why tell this story? Because as a rule, all other things being equal, more power in an amplifier is better. But ultimately it is the efficiency of your loudspeakers that will determine volume level.

Eh? I hear you responding.

Okay, here’s the thing: if you double the power of your amplifier, and if your loudspeakers can cope with the extra power, your maximum volume level will increase by … three decibels. Yes, just three. Double it again and you get three more decibels.

Three decibels is not much. You can hear the difference in level, but it’s not an impressive difference. The reason is that in all kinds of ways our hearing it logarithmic. Each octave is double the frequency range of the one before. Each doubling of power is barely perceptible. That’s not a bug in our hearing, but a feature. If you consider (it’s actually more complicated than this) that the range from the quietest thing you can hear to the loudest thing you can tolerate is 120 decibels, that loud thing is one trillion times more powerful than the quiet thing. If our hearing wasn’t logarithmic, we simply could not cope with the range.

Now, it turns out that loudspeakers are appallingly inefficient. Perhaps it’s for that reason that the normally quoted specification is “sensitivity”, not “efficiency”. The average high-fidelity loudspeaker will have a sensitivity of around 89dB, measured at one metre for an input of 2.83 volts. (Why 2.83 volts? That’s the voltage required to deliver one watt into an eight-ohm load.)

Here’s where it gets embarrassing. The efficiency of such a loudspeaker is 0.5%. That is, 99.5% of the power your amplifier is pouring into it is being turned into heat in the drivers, while just one half of one per cent is being transmitted as acoustic energy into your room.

Each 3dB increase in sensitivity represents a doubling of efficiency. So, 92dB sensitivity is 1% efficient. You can go all the way up to, as far as I can determine, 105dB sensitivity. That’s for the famous Klipschorn loudspeakers, which are enormous three-way, heavily horn-loaded loudspeakers which have been in more or less continuous production since the late 1940s. Back in those days watts were hard to come by and rather expensive, so efficiency was vital. That 105dB sensitivity works out to be 20% efficient.


All of which brings me to this point: don’t fuss too much about your amp’s power output. A few more watts won’t make much difference. What does count is that what it delivers is clean and exercises authority over your loudspeakers, making them do what the amp wants them to do, rather than them doing what they want.

If it’s a choice between 50 and 100 watts, and there’s no reason to think the lower-powered unit is better, then of course go for the extra power. But if you are weighing up amp A, rated at 115 watts per channel, and amp B, rated at 130 watts per channel, just forget about the power output difference. It amounts to an inaudible half a decibel. Instead, consider all the other important things in buying an amp.